Semiconductor chips frequently have between two and seven layers of metal above the silicon. The metal layers provide interconnection to the various transistors in silicon as well as transmit power, clock, and data through various portions of the chip. In order to improve the circuit operation, it is often desirable to have very thick metal layers. Metal layers in excess of 18,000 angstroms, which is 1.8 microns, has particular benefits in certain type of technologies. For example, in certain high-speed, low-resistance applications, inductive coils for RF applications, power distribution, and other circuits, very thick metal is beneficial. Under well known semiconductor deposition processes, it is common to deposit metals in the range of 2,000-6,000 angstroms in thickness. However, depositing metal layers having an overall thickness greater than 15,000 angstroms creates a number of difficulties. As the deposited metal becomes taller on the wafer, defects begin to increase in various parts of the final circuit. In some instances, there are wafer breakages while in other instances, metal shorts may occur. Additional problems are encountered in the chamber which holds the wafer during the metal deposition. In some instances, the metal may be sufficiently thick that a layer of metal at the edge of the wafer connects between the wafer and the clamp holding the wafer such that the wafer sticks to the clamp. This wafer sticking problem leaves a residue on the clamp, and in the worst situations, may connect the wafer to the clamp so that it breaks, or to avoid breakage must physically dislodged in order to remove it. Another problem is the increase in the thermal budget of the chamber during deposition of a thick metal layer which causes many portions of the chamber to be subjected to a higher temperature for longer periods of time than is preferred. Another significant disadvantage is the throughput speed with which thick metal can be deposited. Only a few wafers can be processed at a time and significantly, more time is taken for the thick deposition step with respect to the chamber operation, resulting in a heavy throughput loss.
A further problem is the quality of the metal which is obtained during a thick deposition. With a very thick metal layer, the metal grains are often abnormal and discontinuities resulting in loss of yield due to abnormal grain growth become more frequent. In addition, as time passes during the deposition process the grain growth may vary drastically within the deposition chamber, causing a portion of the metal to have a bad grain structure compared to other portions in the metal layer potentially causing lower conductivity or, in worst cases, potential failure of the circuit during operation.
Despite the shortcomings, people in the industry continue to have the desire to deposit thick metal layers, in excess of 15,000 angstroms, and continue to make efforts to do so.